We use medium-energy ion scattering to determine the composition of each of the topmost three atomic layers of a single crystal of copper coated with an epitaxial submonolayer of nickel as a function of anneal temperature in the range 140-420 K. The technique confers layer and elemental resolution even though the constituents are adjacent in the Periodic Table. At a transition temperature of around 300 K, subsurface copper and surface nickel interchange so that the nickel comes to reside almost exclusively in the subsurface layer, and the top layer of the crystal becomes almost purely copper. Implications of the result for magnetic measurements are briefly sketched.